1. Field of the Invention
The present invention relates to an optical head device, and in particular to an optical head device for the purpose of recording and reproducing in relation to two types of optical recording medium having different substrate thickness.
2. Description of the Related Art
Digital video discs (DVD), which are currently in the process of being developed as a product, have a substrate thickness of 0.6 [mm] as compared 1.2 [mm] in the case of the conventional compact disc (CD). In this situation, there is a demand for an optical head device which will be capable of reproducing both DVDs and CDs.
However, conventional optical head devices are designed in such a manner that the objective lens negates spherical aberration in relation to a disc of a certain thickness. Where a disc of a different thickness is reproduced, spherical aberration remains and it is impossible to reproduce it correctly.
[Conventional Example (1)]
The first example of a conventional optical head device which is capable of reproducing two discs of two different thicknesses is illustrated in FIG. 1 on pp. 460-6 of the Japanese Journal of Applied Physics Volume 36 Part 1 No. 1B. FIG. 19 illustrates the structure of this optical head device. In FIG. 19, a first optical system 111 and a second optical system 112 each have a semiconductor laser which outputs the prescribed laser light, and a photosensor which receives light reflected from the disc (optical recording medium). Of these, the wavelength of the semiconductor laser of the first optical system 111 is 650 [nm], while that of the second optical system is 780 [nm].
Meanwhile, number 113 indicates an interference filter. This interference filter 113 works in such a manner as to transmit light of wavelength 650 [nm], while reflecting light of wavelength 780 [nm]. In this way, light emitted from the semiconductor laser of the first optical system 111 passes through the interference filter 113 and is incident upon a hologram 161. Light which passes through the hologram 161 is incident upon an objective lens 115 in the form of parallel light and converges on a disc (optical recording medium) 116 with a thickness of 0.6 [mm].
Light reflected from the disc 116 passes through the objective lens 115 in the opposite direction and is incident again upon the hologram 161. Light which passes through the hologram 161 passes through the interference filter 113 and is received by a photosensor within the first optical system 111.
Similarly, light emitted from the semiconductor laser of the second optical system 112 passes through the interference filter 113 and is incident upon a hologram 161. First (+) order diffracted light from the hologram 161 is incident upon an objective lens 115 in the form of divergent light and converges on a disc (optical recording medium) 117 with a thickness of 1.2 [mm].
Light reflected from the disc 117 passes through the objective lens 115 in the opposite direction and is incident again upon the hologram 161. First (+) order diffracted light from the wavelength-selective hologram 161 is reflected by the interference filter 113 and is received by a photosensor within the second optical system 112.
The objective lens 115 has a spherical aberration which negates the spherical aberration generated when light of wavelength 650 [nm] emitted from the objective lens 115 passes through a substrate with a thickness of 0.6 [mm], while the hologram 161 has a spherical aberration which negates the sum of the spherical aberration of the objective lens 115 and that which is generated in relation to +1st order diffracted light from the hologram 161 when light of wavelength 780 [nm] emitted from the objective lens 115 passes through a substrate with a thickness of 1.2 [mm].
Consequently, light of wavelength 650 [nm] which passes through the hologram 161 converges as a result of the objective lens 115 without aberration on the disc 116, while +1st order diffracted light of wavelength 780 [nm] converges as a result of the objective lens 115 without aberration on the disc 117.
FIG. 20 presents a top view and a cross-sectional view of the hologram 161.
The hologram 161 is structured in such a manner that a concentric hologram pattern is formed on a glass substrate 118.
Where the cross-section of the hologram pattern 162 is in the form of steps on four levels as in the drawing, and the height of each step is h/2, the refractive index n, and the wavelength of the incident light .lambda., the transmission efficiency .eta..sub.0 and +1st order diffraction efficiency .eta..sub.+1 are given by the following formulae. EQU .eta..sub.0 =cos.sup.2 (.phi./2)cos.sup.2 (.phi./4) (1) EQU .eta..sub.+1 =(8/.pi..sup.2)sin.sup.2 (.phi./2)cos.sup.2 [(.phi.+.pi.)/4] (2) EQU where, .phi.=2.pi.(n-1)h/.lambda. (3)
For instance, when h=2.83 [.mu.m] and n=1.46, since .phi.=4.pi. for .lambda.=650 [nm], .eta..sub.0 =1, .eta..sub.+1 =0.
Similarly, since .phi.=3.33 .pi. for .lambda.=780 [nm], .eta..sub.0 =0.188, .eta..sub.+1 =0.567.
In other words, light of wavelength 650 [nm] emitted from a semiconductor laser all passes through the hologram 161 and heads towards the disc 116, while 56.7% of light of wavelength 780 [nm] emitted from a semiconductor laser is diffracted by the wavelength-selective hologram 161 as +1st order diffracted light and heads towards the disc 117.
Moreover, as FIG. 20 shows, if the effective diameter of the objective lens 115 is 2a, the hologram pattern 162 is formed only within an area 2b of a diameter smaller than this. Outside the area of diameter 2b, light of wavelengths 650 [nm] and 780 [nm] all passes through the hologram 161.
In other words, with the hologram 116, light of wavelength 650 [nm] all passes through, while 56.7% of light of wavelength 780 [nm] is diffracted within the area of diameter 2b as +1st order diffracted light, and none is diffracted outside the area of diameter 2b, where 2a and 2b are the diameters shown in FIG. 20(a).
Consequently, if the focal length of the objective lens 115 is f, the effective numerical aperture in relation to light of wavelengths 650 [nm] and 780 [nm] is given by a/f and b/f respectively. For example, if f=3 [mm], a=1.8 [mm] and b=1.35 [mm], a/f=0.6 while b/f=0.45.
[Conventional Example (2)]
The second example of a conventional optical head device which is capable of reproducing two discs of two different thicknesses is illustrated in FIG. 7 on pp. 460-6 of the Japanese Journal of Applied Physics Volume 36 Part 1 No. 1B. FIG. 21 illustrates the structure of this optical head device (conventional example 2).
In FIG. 21 also, a first optical system 111 and a second optical system 112 each have a semiconductor laser, and a photosensor which receives light reflected from the disc. The wavelength of the semiconductor laser of the first optical system 111 is 650 [nm], while that of the second optical system is 780 [nm]. The interference filter 113 works in such a manner as to transmit light of wavelength 650 [nm], while reflecting light of wavelength 780 [nm].
Light emitted from the semiconductor laser of the first optical system 111 passes through the interference filter 113 and an aperture 163 to be incident upon the objective lens 115 in the form of parallel light and converge on the disc 116, which has a thickness of 0.6 [mm]. Light reflected from the disc 116 passes in the opposite direction through the objective lens 115, aperture 163 and interference filter 113, and is received by the photosensor within the first optical system 111. Meanwhile, light emitted from the semiconductor laser of the second optical system 112 is reflected by the interference filter 113, and passes through the aperture 163 to be incident upon a objective lens 115 in the form of divergent light and converge on the disc 117, which has a thickness of 1.2 [mm].
Light reflected from the disc 117 passes in the opposite direction through the objective lens 115 and aperture 163, is reflected by the interference filter 113, and received by the photosensor within the first optical system 111.
The objective lens 115 has a spherical aberration which negates the spherical aberration generated when light of wavelength 650 [nm] emitted from the objective lens 115 passes through a substrate with a thickness of 0.6 [mm].
When light of wavelength 780 [nm] which is incident upon the objective lens 115 in the form of parallel light passes through a substrate with a thickness of 1.2 [mm], the spherical aberration remains.
However, when light of wavelength 780 [nm] is incident upon the objective lens 115 in the form of divergent light, new spherical aberration is generated accompanying object point movement of the objective lens 115, and this works in the direction of negating the spherical aberration which remains when it passes through a substrate with a thickness of 1.2 [mm].
Consequently, if the object point position of light of wavelength 780 [nm] is set at its optimum, light of wavelength 650 [nm] converges without aberration on the disc 116, while light of wavelength 780 [nm] converges without aberration on the disc 117.
FIG. 22 (a), (b) present a top view and a cross-sectional view of the aperture member 163. This aperture member 163 is structured in such a manner that an interference filter pattern 120 is formed on a glass substrate 118.
If the effective diameter of the objective lens 115 is 2a, the interference filter pattern 120 is formed only outside an area of a diameter 2b smaller than this.
The interference filter pattern 120 not only serves to allow all the light of wavelength 650 [nm] to pass through and all the light of wavelength 780 [nm] to be reflected, but also serves to adjust to an integral multiple of 2.pi. the phase difference between the light which passes through within the area of diameter 2b and that which passes through outside it. In other words, with the aperture member 163, light of wavelength 650 [nm] all passes through within the area of diameter 2a, while all the light of wavelength 780 [nm] passes through within the area of diameter 2b, and is all reflected outside that area.
Consequently, if the focal length of the objective lens 115 is f, the effective numerical aperture in relation to light of wavelengths 650 [nm] and 780 [nm] is given by a/f and b/f respectively.
For example, if f=3 [mm], a=1.8 [mm] and b=1.35 [mm], a/f=0.6 while b/f=0.45.
[Conventional Example (3)]
The third example of a conventional optical head device which is capable of reproducing two discs of two different thicknesses is disclosed in Japanese Patent H6[1994]-295467. This optical head device has a variable phase plate 164 between the semiconductor laser and the objective lens. The variable phase plate 164 is structured in such a manner that a ring-shaped substrate 165, sandwiched between electrodes 166, is formed on a glass substrate 118.
The ring-shaped substrate 165 has the property that its refractive index changes according to the electric field, so that by altering the voltage impressed on the electrodes 166 it is possible to change the phase difference of the light passing through the interior and exterior of the ring-shaped substrate 165.
FIG. 24 is a diagram which illustrates the properties of wave front aberration in this optical head device.
The horizontal axis represents the wave front aberration normalized by the wavelength of the light, while the vertical axis represents the distance from the optical axis normalized by the focal length of the objective lens, i.e. the numerical aperture.
Taking the wavelength of the light as 670 [nm], the numerical aperture of the objective lens as 0.6, and the difference in substrate thickness in relation to the design value as +0.1 [mm]. The focus is controlled in such a manner that wave front aberration of a beam of numerical aperture 0.6 is 0.
FIG. 24 (a) illustrates an example where no phase difference is given by the ring-shaped substrate 165, and the standard deviation of the wave front aberration is 0.095 .lambda.. Meanwhile, FIG. 24 (b) illustrates an example where a ring-shaped substrate 165 with an internal numerical aperture of 0.244 and an external numerical aperture of 0.560 has given a phase difference of 0.316 .pi., and the standard deviation of the wave front aberration is reduced to 0.048 .lambda..
In the first example of a conventional optical head device, light of wavelength 650 [nm] all passes through the hologram 161, while 56.7% of light of wavelength 780 [nm] is diffracted by the hologram 161 as +1st order diffracted light.
This results in the disadvantage that it is impossible to obtain a good S/N when reproducing the disc 117, or to obtain satisfactory optical output when recording.
In the second example of a conventional optical head device, light of wavelength 650 [nm] is incident upon the objective lens 115 in the form of parallel light, while light of wavelength 780 [nm] is incident in the form of divergent light. As a result, the light of wavelength 650 [nm] does not generate any aberration when the objective lens 115 is driven by means of the actuator in the focusing or tracking direction, but the light of wavelength 780 [nm] does.
This results in the disadvantage that it is impossible to obtain a good S/N and jitter N when reproducing the disc 117, or to obtain satisfactory peak intensify when recording.
The light in the third example of a conventional optical head device is of a single wavelength. It is possible to reproduce a digital video disc at wavelength 650 [nm], but not at 780 [nm] because it is impossible to obtain a sufficiently small light spot diameter.
Conversely, it is possible to reproduce a recordable compact disc at wavelength 780 [nm], but not at 650 [nm] because it is impossible to obtain a sufficiently reflecticity. This results in the disadvantage that recordable compact discs cannot be reproduced if the wavelength of the light is 650 [nm].